1. Field of the Invention
The present invention relates to a compound semiconductor device and a method of manufacturing the same, and particularly, to a compound semiconductor device involving a small number of processes to efficiently protect the top of layers grown thereon and a method of manufacturing such a compound semiconductor device.
2. Description of the Related Art
Various types of compound semiconductor devices have been actively developed FIG. 6 is a sectional view showing a blue light emitting diode as an example of a gallium-nitride-based compound semiconductor device. This device is composed of a sapphire substrate 101 and layers grown thereon. The layers include a GaN buffer layer 103, an n-GaN layer 105, an InGaN active layer 107, a p-AlGaN layer 109, and a p.sup.+ -GaN contact layer 111.
The top layer 111 is covered with a passivation film 113, which is an insulating or dielectric thin film made of, for example, SiO.sub.2 or SiNx.
A method of manufacturing the device of FIG. 6 will be explained. Firstly, the sapphire substrate is set in a reaction chamber of an MOCVD (metal organic chemical vapor deposition) apparatus. In the reaction chamber, the layers 103 to 111 are epitaxially grown on the substrate. The substrate 101 with the layers 103 to 111 is cooled in the reaction chamber with a carrier gas and a V-group gas such as an ammonia gas being supplied to the reaction chamber. After the substrate 101 is sufficiently cooled, it is taken out of the reaction chamber.
The top of the layers 103 to 111 is lightly etched, and the substrate 101 with the layers is transferred into a separate thermal or plasma CVD apparatus in which the passivation film 113 is formed on the top layer 111.
If the separate apparatus is a thermal CVD apparatus and the passivation film 113 includes SiO.sub.2, the substrate 101 is heated in the apparatus to a temperature in the range of 350.degree. C. to 500.degree. C. to form the thin film 113. If the apparatus is the same and the thin film 113 includes SiNx, the substrate 101 is heated in the apparatus to a temperature in the range of 800.degree. C. to 1200.degree. C. to form the thin film 113. If the separate apparatus is a plasma CVD (P-CVD) apparatus, the substrate 101 is heated to a temperature in the range of 100.degree. C. to 300.degree. C. in a vacuum formed in the apparatus.
After the formation of the passivation film 113, the substrate 101 with the layers 103 to 111 and thin film 113 (collectively referred to as the product) is cooled in the apparatus. When the product is sufficiently cooled, it is taken out of the apparatus. The product is then transferred to an annealing apparatus, which anneals the product in a nitrogen atmosphere, to further activate p-type impurities added among the grown layers and realize good p-type conductivity.
The passivation film 113 mainly functions to protect the top of the layers 103 to 111 against oxidization, contamination, flaws, etc. In III-V-group compound semiconductor in particular, V-group elements have a high vapor pressure to cause an escape of As from GaAs-based material, an escape of P from InP-based material, an escape of N from GaN-based material, and an evaporation of the V-group elements. The thin film 113 prevents such escape and evaporation.
The conventional manufacturing method, however, must employ separate apparatuses to epitaxially grow the layers 103 to 111 and form the passivation film 113. When the substrate is transferred from the MOCVD apparatus to the thermal CVD apparatus after the growth of the layers 103 to 111, the top of these layers is exposed to open air and is oxidized thereby. If the top of the layers contains material such as Al that is easily oxidized or nitrided, the oxidization or nitriding starts as soon as the substrate is taken out of the apparatus. The oxidization and nitriding cause defects to deteriorate the reliability of the compound semiconductor device thus formed.
To solve this problem, the top of the layers 103 to 111 is lightly etched before the formation of the passivation film 113, as mentioned above. This etching, however, may adversely roughen and contaminate the top of the layers.
When forming the passivation film 113, the substrate with the grown layers must be set in a separate apparatus and must be heated to a predetermined temperature. During the heating process, the top of the grown layers is bare, and therefore, V-group elements may escape therefrom. In particular, when the heating process is carried out in a vacuum, a vapor pressure increases to easily make V-group elements escape and cause crystal defects.